The present invention involves the continuous production of rubber strip material impressed with a surface design and, more particularly, to an apparatus and process for the molding of a continuous strip of uncured or partially cured rubber with a design, such as a tire tread, and the curing thereof after removal from the mold.
Presently, tire treads, such as are employed in retreading worn tires, are made for the most part by simultaneously shaping and heat curing rubber compounds in a mold. The rubber stock is kept in the mold under pressure for essentially the entire curing period, since premature removal may cause so-called "blowing of the stock", that is, formation of numerous small voids caused by expansion of water and gases generated during the curing process. High pressures and long molding times of over 15 minutes are generally required to prepare a tread of good quality. As a result, this process has been essentially discontinuous, that is, the treads have been made in individual sections one at a time. Even when the process has been made continuous using cooperating endless belt molds with the rubber in strip form, such as disclosed, for example, in U.S. Pat. Nos. 3,214,792, 3,827,846 and Ger. No. 71 10 999, it has been found to be comparatively time-consuming.
Attempts at using more rapid radiation curing of the tire tread rubber, such as suggested, for example, in U.S. Pat. Nos. 1,906,402, 2,933,441, and 3,959,053, have not been entirely successful. It has been thought impractical to completely cure such treads by radiation, as it would require an exposure of the rubber stock while traveling in the mold. Penetration of the mold, which is usually made from steel, would necessitate the use of gamma rays, and as commercial sources produce gamma rays only at moderately low intensities, very long tread cure times would be required.
It has now been appreciated by the present inventors that tire treads can be rapidly produced and completely cured by radiation if they are first molded and then removed from the mold in the uncured state without causing a distortion of the intricate tread pattern. The undistorted removal of the uncured tread had not been thought possible by those skilled in the art. The present invention provides an apparatus and process which take advantage of this heretofore unrealized capability of uncured or green rubber to be molded without curing and uses subsequent radiation or heat curing after the removal of the embossed rubber from the mold to produce an improved product at a more rapid rate.
Further, in the past, the time required for molding the rubber has been determined by curing conditions and not mold design. Such factors as heat transfer, prevention of blowing, modulus development, and induction time have required molding cycles to be longer than the time actually needed to achieve mold design definition. Because of the fact that curing is not carried out in the mold, heat transfer and blowing problems are avoided, and a more rapid cure with radiation outside the mold may be used to achieve fast modulus development. Molding time with the present invention therefore is dependent only on the conditions necessary to produce the desired design on the green tread in the mold, and to retain the design outside of the mold prior to radiation or heat curing. As a result, the molding and curing times are considerably reduced. Molding cycles can be under 5 minutes at 120.degree. C. and 1000 psi for fast molding, and this permits the design of equipment for continuous and rapid production.